1. Field of the Invention
This invention relates generally to the field of formation subsidence measurements and is more particularly concerned with apparatus of this type to be used with a buried radioactive source.
2. Prior Art
When a reservoir is produced and its fluid pressure falls, some of the overburden stress is transferred to the adjacent rock matrix. This increased stress causes the rock to compact both elastically and inelastically. If compaction is great enough, this can lead to undesirable or dangerous subsidence effects. Similar problems arise as a result of the production of a coal seam.
A known technique for measuring formation subsidence employs a radioactive source such as a cesium pellet implanted, buried or fired into a formation of interest as a depth marker. Formation subsidence is then measured by detection of the radiation from such source with a downhole logging tool.
This prior art technique involves lowering a gamma detector on a cable in a borehole adjacent to the source and recording the fluctuating gamma ray intensity as the sonde logs over a prescribed distance. The logging operation may be repeated at intervals to observe if a shift occurs in the depth at which maximum signal is observed.
A shortcoming of the above method is that customarily tubing collar location is employed as a reference point for source movement. Therefore the borehole must be cased. Furthermore, if the location of the collar used as a reference has changed through subsidence, subsequent radioactive signal indications will no longer be reliable. This can prevent accurate measurement of very small plastic rock deformations, say on the order of several hundredths of an inch. Changes of this magnitude can be significant precursors of major changes.
A related disadvantage inherent in the above-described prior art technique lies in the fact that it is discontinuous. Depending upon the time intervals between successive logs it may, therefore, be ineffective as an early warning device.
A further disadvantage of prior art logging devices of the type described is that the reliability of signal measurements is degraded by cable tension oscillation and twist. In the short term, when a cable is lowered and a few logging runs are conducted, the signal readings from one scan to the next may change so as to give false indications.
It is, therefore, a general object of this invention to provide an improved logging type subsidence detector employing a radioactive source.
It is a further object of this invention to provide a logging type subsidence detector which permits early warning of major subsidence events.
Other objects and advantages of this invention will become apparent from a consideration of the detailed drawings and description to follow.